Mold wash



United States Patent 3,121,269 MGM) WASH Paul J. Nefi, .in, Lansing,Ill, assiguor to Amsted industries Incorporated, tlhieago, Ill, acorporation of New Jersey No Drawing. Filed Nov. 25, 196%, Ser. No.71,446 ll Claim. (Cl. 22--2l6.5)

This invention relates to shell molding and particularly to theelimination of surface defects in low carbon, low alloy steel castingsproduced in conventional shell molds.

Shell molds, as well known in the art, are generally formed of a mixtureof zircon or silica sand and a thermosetting phenolic resin either inpowder form or in a solvent. If desirable, dust suppressants may be usedand in some instances a release agent may be incorporated into themixture to prevent the shell from adhering to the pattern which isheated during the forming of the shell to set the resin.

Although such molds ordinarily produce commercially acceptable ironcastings and stainless steel castings, it is well known that low carbon,low alloy steel castings, such as those formed, for example, of grade Bsteel, are subject to various sunface defects when formed in such molds.The so called characteristic surface defects of such castings, such asblow holes, porosity, pock marking and the like, are generallycompletely eliminated by the utilization of a mold Wash comprisingcertain metal oxides as disclosed and claimed in US. Letters Patent2,847,741. It has been found, however, that the metal oxide washes,while effective to eliminate characteristic shell mold defects, were notcompletely effective to eliminate other types of defects. For example,when structures were cast having configurations which tended to formlocalized hot spots due to unequal sectional thicknesses, or to themerging of metal sections, the metal surface at the localized areascontained a roughness comprising globular protuberances or What arecommonly called positive type surface defects.

Accordingly, the primary object of the present invention is theelimination of surface defects from low carbon, low alloy steel castingformed in shell molds, particularly those surface defects which are ofthe positive type.

According to the invention, it is theorized that the roughness, orpositive type defects, are caused by a recarburization efiect inlocalized areas after the formation of a metal skin. Such carburizationlower the melting point of the metal. As a consequence, the metal whichhad solidified and formed the skin remelts and then bleeds or dripsbecause the bulk of the metal is cooling, contracting, and pulling awayfrom the mold.

According to the present invention, when a highly oxidizing wash isapplied to the surface of the shell mold, the carburizing and consequentremelting of the metal apparently do not occur and the positive typesurface defects together with the typical shell mold defects areelfectively eliminated. In view of the fact that the mold wash isgenerally applied to a warm mold, which may be at a temperature ofapproximately 200 B, it is preferable that the agent utilized for a washdoes not decompose or liberate oxygen at a temperature of less than 250F. Of course, if an unheated mold is to be coated, the decomposition oroxygen liberating temperature may be as low as slightly above roomtemperature. It is essential 3,l2l,2 Patented Feb. 18, 1954 that thedecomposition temperature of the wash, either alone or in the presenceof a catalyst, is less than the melting point of the metal to be pouredand, preferably, the decomposition temperature should be less than about1500" F. in order that the oxygen will be liberated quickly enough toaccomplish its intended purpose.

The oxidizing agent, in order to be applied to a shell mold surface insufiicient quantities to be effective, must be rather highly soluble inWater or in other solvents, such as for example, alcohol. It isprefer-able also that oxidizing agent should be non-toxic upondecomposition and for practicability should be commercially available ata reasonable cost.

The following table sets forth various chemical agents which were coatedon shell molds in which were poured low carbon, 10W alloy steelcastings. The table also sets forth the decomposition temperature, thesolubility and the effectiveness of each agent in eliminating positivetype surface defects.

The above table indicates clearly that the effectiveness of the agentutilized for a wash lies in the release of oxygen at the time of castingwhich influences the mold atmosphere in such a manner, apparently byconverting the carburizing carbon monoxide to non-carburizing carbondioxide, that carburization and remelting of the cast metal cannotoccur. For example, agent number 8, namely, sodium borate, might beexpected to be as effective as agent number 3, sodium nitrate. However,it will be noted that sodium borate is only slightly soluble as comparedto sodium nitrate and, for this reason, it is impossible to apply asuflic-ient amount of sodium borate to the mold surface to provide aneffective quantity of oxygen. Similarly, it might :be expected thatagent No. 7, potassium perchlorate, would be more effective than agentNo. 6, potassium chlorate, due to the fact that the former has a greateramount of oxygen available for a given weight of the agents. Once again,however, the potassium perchlorate, having extremely low solubility, wasless effective. The potassium chlorate, having a higher solubilityrating, was fairly effective but still was less effective than the firstfive agents listed, all of which were much more soluble. It will benoted also that agents 10 and ll, namely manganese dioxide and titaniumdioxide, which were effective in eliminating characteristic defects, arepractically insoluble and also ineffective to eliminate positive typedefects.

The oxygen releasing agents are effective for the designated purposewhen used alone in a suitable solvent. iowever, it may be desirable thatother substances also be used either to add coloring or opaqueness tothe coating solution or to eliminate side eifects which at times may beundesirable. For example, fluxing, or lowering of the fusion point ofthe sand in the shellmold, may occur. This may be prevented by theutilization in the wash of a suitable ceramic material, such as titaniumdioxide, iron oxide, manganese dioxide or the like.

Several typical Washes are set forth below, by Way of example only, toillustrate the various combinations of additives which may be utilizedwith the oxygen releasing agents and also to illustrate the Wide rangein strength of solution which is effective.

Wash No. 1

KClO "grams" 10 E cc 150 MnO grams 100 Cellulose gum gram .75 Wash No.2:

LiNO grams 52 H O ml 100 Alcohol ml 50 Wash No. 3:

LiClOn; g-rams 10 Ti0 do 10 E 0 ml 100 Wash No. 4:

KCIO gran1s 1-0 TiO do 50 H O ml 150 Wash No. 5:

LiClO grams 1O H O ml 100 Wash No. 6:

LiClO4 grams 60 E 0 ml 100 Wash No. 7:

LiClO grams 60 MnO do 100 H2O In1 Wash No. 8:

NaNO grams 60 E 0 do 100 Wash No. 9:

LiClO "grams" 45 E 0 cc 100 Wash No. 10:

LiClO grams 45 TiO do 30 E 0 cc 100 Wash No. 11:

LiClO grams 45 M1102 do 30 "E0 do 30 H O cc 100 Wash No. 12:

LiClO "grams" 30 Ti0 do M110 do 20 H2O CC Wash No. 13:

Mg(ClO grams 50 H O cc Wash No. 14:

LiClO grams 36 Ti0 do 24 Fe O do 24 E 0 cc 100 Wash No. 15:

Mg(ClO grams 25 H O cc 100 The agents which were selected and utilizedin the formulations set forth in the above table are those which releaseoxygen on heating or in the presence of a catalyst, which areinexpensive, and which are readily available commercially. With regardto the use of a catalyst it will be noted that wash No. 1 was slightlymore effective than Wash N0. 4. The reason for this is that themanganese dioxide in Wash No. 1 acts as a catalyst to release oxygenfrom the perchlorate, whereas the titanium dioxide utilized in Wash No.4 does not. It should also be noted that wash No. 6, which containslithium perchlorate to the limit of its solubility, was slightly moreeffective than wash No. 5, thereby showing the desirability of havingthe largest amount out oxygen available in order to effectively preventclarburization.

The Wash may be applied to the casting surface of the mold cavity bybrushing, but preferably, in order to obtain a uniform coating, the washshould be sprayed onto the mold. The controlling factor for thequantities of solid materials which are suspended in the solution is theviscosity of the wash, which must be such that the wash can be sprayedwith conventional spraying equipment.

It has been found that the Wash soaks into the mold to such an extentthat it is difiicult to apply too heavy a coating. The actual amount ofWash which is utilized is not critical, and the wash may be sprayeduntil it begins to run on the mold surface. The wash should not be sothickly applied that undue cooling of the shell occurs because it isdesirable that the wash should dry and the mold he usable severalseconds after application of the Wash.

The sequence of steps in preparing the mold Wash also is not critical.However, it is preferred that the oxidizing agent first be dissolvedwhile agitating the solvent. The solids are then added While continuingto agitate the solvent. If desired, an emulsifying agent may 'beutilized to hold the solids in suspension.

1 claim:

A method of producing low carbon, low alloy steel castings consistingessentially of: making a shell mold having sand particles bondedtogether with a thermosetting resin, then coating the mold cavity with asolution of a metallic compound selected from the group consisting ofnitrates, chlorates and perchlorates, and having a solubility rating ofat least about 7 grams per milliliters of cold Water, and then fillingthe mold with molten metal.

Budd May 10, 1870 Meves et al Aug. 19, 1958

